A ball is thrown up from the top of a tower at 20 m/s velocity. It took the ball 6 sec to fall to the bottom. How high is the tower? For how long was the ball above the tower?
When the ball was going up till B then its displacement was increasing.
When the ball started to fall from B to C its displacement was still positive but decreasing when ball is at C its displacement from the starting position is 0.
So when the ball comes back to C after falling from B then its displacement becomes O
So, we know ball was thrown with 20 m/s up
u = + 20 m/s
s = 0
Acceleration = - 10 m/s2
As always the bell is under the influence of gravity
t(20 - 5t) = 0
t = 0 or 4 sec
The equation gives 2 answers for t at which s = 0
By analyzing we see that at t = 0 particle was at starting point only so displacement = 0
At, t = 4 s = 0
Means ball comes back to starting point after its journey.